1. Field of Invention
This invention relates to a switching power supply used, for example, in electronic equipment, such as computers; and more particularly, to such power supply that can start or stop a primary switching element thereof by using a start or stop signal from a secondary circuit, or by using a command from an over-voltage protection circuit.
2. Description of the Prior Art
Generally, in conventional switching power supplies, two or three isolators are used to isolate a feedback signal for output voltage stabilization, to isolate the signal from an over-voltage protection circuit for the output voltage, and to isolate a remote control signal for turning ON and OFF power with a secondary external switch. In such power supplies, photo-couplers or transformers are used as the isolators. For example, in Japan Published Unexamined UM application HEI 1-79,389, a remote control signal and an over-voltage protection signal are isolated using only one isolator. However, since the isolator is separately provided, to isolate the feedback signal for out-put voltage stabilization, two or more isolators are used for the entire power supply. In such a case, a problem arises in that the number and cost of components are increased. Another problem is that if isolation is intended between the primary and secondary circuits, using photo-couplers or transformers complicates the design of printed wiring boards because of creepage distances and clearances due to limitations in various standards, such as safety standards, must be taken into consideration.
In Japan Published Unexamined Patent application Hei 4-156,270 a power supply is disclosed using one isolator to isolate three types of signals. However, this power supply has a problem in that since the latch circuit for over-voltage protection is included in the primary circuit, it is in possible to release the latch and restart the power supply using a remote control signal. Also, another problem is that the commercial power line voltage must be turned OFF to release the latch. Thus, the turning OFF procedure cannot be applied to some types of applications of the power supply. In order to solve these problems, a switching power supply having the circuitry shown in FIG. 1 was proposed.
In FIG. 1, main converter 10 outputs main output voltage Vo to a secondary circuit by turning ON and OFF a DC current applied to the primary winding n1 of the main transformer T1 using main switching element Q1 and rectifying and smoothing the current induced in the secondary winding n2. Since a commercial AC power line is used as an input source in the power supply, DC input voltage Vidc is obtained by rectifying and smoothing the input by using diode bridge DB and capacitor C1. Although an FET (field effect transistor) is used for the main switching element Q1, either a PNP or NPN transistor can also be used. As the secondary rectifying and smoothing circuit, the anode terminals of diodes D1 and D2 are connected to each end of secondary winding n2, respectively, and the cathode terminals of both diodes are short circuited and connected to output capacitor C2.
Error amplifier 20 generates an error voltage between main output voltage Vo and first reference voltagae Vref1 and, in this case, uses operational amplifier U1. Main output voltage Vo is divided in a suitable table ratio using dividing resistors R1 and R2, and sent to error amplifier 20. PWM (Pulse Width Modulation) circuit 30 outputs a drive signal for the main switching element to minimize an error voltage outputted by error amplifier 20, and uses, in this case, a PWM control circuit U2. Auxiliary converter 40 turns ON and OFF the DC current applied to primary winding n3 of auxiliary transformer T2 using auxiliary switching element Q2. The current induced in secondary winding n4 of the primary circuit is outputted as auxiliary voltage Vcc1 for PWM circuit 30 via the rectifying and smoothing circuit comprising diode D4 and capacitor C4. The current induced in the secondary winding n5 of the secondary circuit is outputted as secondary auxiliary voltage Vcc2 via the rectifying and smoothing circuit comprising diode D3 and capacitor C3.
Photo-coupler PC receives an error voltage outputted from the error amplifier 20 as an input signal on the light emission diode side and outputs an output signal from the photo transistor detector to the PWM circuit 30. Over-voltage protection circuit 50 generates an over-voltage protection signal when main output voltage Vo exceeds the second reference voltage Vref2, and uses, in this case, operational amplifier U3. Since main output voltage Vo is grounded through a series circuit comprising Zener diode D5 and resistor R4, the voltage generated across resistor R4 is inputted to the positive terminal of operational amplifier U3.
Power ON/OFF circuit 60 outputs either secondary auxiliary voltage Vcc2 or the common potential depending on the remote control signal sent from the secondary circuit. In this case, operational amplifier U4 receives a potential at the terminal of resistor R5, on the switch SW1 side, as an input signal, at its positive terminal and also receives third reference voltage Vref3, as an input signal, at the negative terminal. Operational amplifier U4 outputs a high (H) or low (L) level signal corresponding to the remote control signal ON or OFF. Switch SW1 comprises a contact switch or a TTL circuit and defines the ON and OFf status of the remote control signal. When the remote control signal is ON, switch SW1 is closed and as a result the output of operational amplifier U4 is changed to L.
Shut-down latch circuit 70 receives the over-voltage protection signal from over-voltage protection circuit 50, at the set terminal thereof as an input signal, and receives the remote control signal from power ON/OFF circuit 60, at the reset terminal thereof as another input signal. Shut-down latch circuit 70 outputs the shut-down signal from the output terminal thereof. In this case, the shut down latch 70 comprises an RS flip-flop U5. Shut-down execution circuit 80 retains the input voltage for photo-coupler PC at a low level (L) when either of the remote control signal from power ON/OFF circuit 60 and the shut-down signal from the shut-down latch circuit 70 indicates power OFF.
In shut-down execution circuit 80, the output signal from operational amplifier U4 and the output signal from shut-down latch circuit 70 are inputted to OR circuit U6 and an output signal from OR circuit U6 turns ON and OFF switch SW2. When switch SW2 is turned ON, input voltage VPD of photo-coupler PC is maintained at state L. When switch SW2 is turned OFF, the input voltage VPD of the photo-coupler PC is equal to the output voltage E/AOUT from error amplifier 20.
Operation of the FIG. 1 circuit will be described with reference to the waveform chart of FIG. 2, wherein line (A) shows remote control signal ON and OFF; line (B) show the output from the shut-down latch circuit 70; line (C) shows the input voltage VPD and the output voltage VFB of the photo-coupler; line (D) shows the main switching element Q1 driving signal outputted from the PWM circuit; and line (E) shows the main output voltage Vo. Auxiliary converter 40 is always operating regardless of whether the remote control signal is ON or OFF as long as the commercial power AC voltage in supplied to the circuit. It is assumed that auxiliary converter 40 is already started by turning ON the commercial power AC voltage and the PWM circuit 30 driving voltage Vcc1 and the driving voltage Vcc2 for the secondary control circuit are supplied in a stable manner.
At time T1, the remote control signal changes from an OFF state (high level H) to an ON state (low level L). Then, the output signal from operational amplifier U4, in power ON/OFF circuit 60, changes to a low state L and switch SW2 is opened. Then, the output voltage from operational amplifier U1 of error amplifier 20 is supplied as an input signal voltage VPD to the photo-coupler PC. On the other hand, when the output voltage VFB of the photocoupler is equal to or less than the predetermined threshold voltage Vth, the PWM circuit 30 does not output pulses from the OUT terminal thereof. As the output voltage VFB becomes higher than the threshold voltage Vth, PWM circuit operates so that the duty ratio thereof of the ON and OFF states becomes large. Thus, when error voltage signal E/AOUT of error amplifier 20 starts to be transmitted from the photo-coupler PC, emitter voltage VFB of the photo transistor detector rises from 0 volts. When VFB exceeds threshold voltage Vth, the driving signal for the main switching element Q1 is outputted from the OUT terminal of the PWM circuit. Then, main converter 10 starts operation, and the main voltage Vo begins to rise.
In addition, in FIG.2, the input voltage VPD and output voltage VFB of the photocoupler PC and main output voltage Vo rise with a slope, respectively. This is because mounting of a so-called slow start circuit is considered in the secondary circuit. That is, although the rush current is prevented from occuring when the slow start circuit is provided, error voltage signal E/AOUT from error amplifier 20 increases gradually at the time of start up.
Next, when the remote control signal is turned OFF at time T2, the output signal from operational amplifier U4 of power ON/OFF circuit 60 is changed to a high state H and switch SW2 is closed to clamp input voltage VPD from the photo-coupler to a low state L. Then, since the output voltage VFB from the photo-coupler drops to the predetermined threshold voltage Vth or less, the duty ratio of the ON state of the main switching element Q1 driving signal outputted from the OUT terminal of the PWM circuit 30 becomes zero to stop the operation of the main converter 10.
Next, assume that an over-voltage is generated in the main output voltage Vo at time T3. Then, over-voltage protection circuit 50 detects the over-voltage and sets shut-down latch circuit 70. This clamps input voltage VPD of the photo-coupler to a low state L, and stops operation of the main converter 10 until the shut-down latch circuit 70 is reset. While the main converter 10 is in a non-operated state, the defect that caused the over-voltage is removed.
When the remote control signal is turned OFF at time T4, the out-put signal from the operational amplifier U4 in power ON/OFF circuit 60 is changed to a high state H, and shut-down circuit 70 is reset. After that, when the remote control signal is turned ON at time T5, the main converter 10 is restarted.
Since an over-voltage protection signal and a remote control signal are superimposed on the feedback signal of the output voltage, only one photo-coupler, that is the isolator, is required. Thus, the conventional switching power supply shown in FIG. 1 has the advantage of low cost, and good reliability. However, the FIG. 1 system also has the following problems. The feedback signal of the output voltage, which is obtained by superimposing an over-voltage protection signal on a remote control signal, is transmitted to the primary circuit by use of only one photo-coupler. Thus, if an abnormal status, such as increase in output signal due to a failure in the photo-coupler, the conventional power supply cannot be stopped and an over-voltage could continue to be supplied to the load.